Think I've used about 12/13g for things like mild or bitters to get that low level of carbonation that I'd expect from a cask ale. I base this on one of the online priming calculators, targeting 1.5 volumes of co2.
For a more recent neipa I used something more like 20g to get increases carbonation but haven't tasted it yet. I'm not sure I'd go much higher than that though, for fear of bursting the keg!
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